The rate of tumor growth will be measured in host animals that have been actively immunized with immunogenic conjugates containing prostaglandins E2, F2alpha and their metabolites. The transplantable tumors will include mammary adenocarcinomas, anaplastic carcinomas, melanomas, sarcomas, hepatomas, and neuroblastomas carried in the appropriate strain of mice; Walker carcinosarcoma transplanted in the appropriate strain of rats; V2 carcinoma in the rabbit; myofibrosarcoma in the hamster; and Rous sarcoma in chickens. The initial studies will be done with mouse tumors. The rate of growth of the tumors, which will be measured by weight of the tumor excised at various times after transplantation and/or death of the host, will be correlated with the binding constants of the various prostaglandins and the circulating antibodies of the host.